Editor's note: In part one in this multipart series of excerpts from Practical Unix & Internet Security, 3rd Edition, we covered tips and general design principles to code by to help you avoid security-related bugs. This week we offer advice both on what to do and what not to do when you are coding a new network program. And check back to this space over the next few weeks for tips on writing SUID/SGID programs, and on using passwords to generate random numbers, all from Chapter 16 on "Secure Programming Techniques."
If you are coding a new network service, there are a number of pitfalls you will need to consider.
Do a reverse lookup on connections when you need a hostname for any reason. After you have obtained a hostname to go with the IP address you have, do another lookup on that hostname to ensure that its IP address matches what you have.
Include some form of load shedding or load limiting in your server to handle cases of excessive load. Consider what should happen if someone makes a concerted effort to direct a denial-of-service attack against your server. For example, you may wish to have a server stop processing incoming requests if the load goes over some predefined value.
Put reasonable timeouts on each network-oriented read request. A remote server that does not respond quickly may be common, but one that does not respond for days may hang up your code awaiting a reply. This rule is especially important in TCP-based servers that may continue to attempt delivery indefinitely.
Put reasonable timeouts on each network write request. If some remote server accepts the first few bytes and then blocks indefinitely, you do not want it to lock up your code awaiting completion.
Make no assumptions about the content of input data, no matter what the source is. For instance, do not assume that input is null-terminated, contains linefeeds, or is even in standard ASCII format. Your program should behave in a defined manner if it receives random binary data as well as expected input. This is especially critical on systems that support locales and that may get Unicode-formatted input.
When checking the content of input, try to validate it against acceptable values, and reject anything that doesn't match what's allowed. The alternative (and all too common) strategy of rejecting invalid values and allowing anything else requires you to specify (and in some cases, predict) all of the possible invalid values that might arise, ever.
Make no assumptions about the amount of input sent by the remote machine. Put in bounds checking on individual items read, and on the total amount of data read.
Consider doing a call to the authd service on the remote site to identify the putative source of the connection. However, remember not to place too much trust in the response, and to build in a timeout in the event that you don't get an answer.
Consider adding some form of session encryption to prevent eavesdropping and to foil session hijacking. But don't try writing your own cryptography functions; see Chapter 7 ("Cryptography Basics") for algorithms that are known to be strong. Using SSL builds on known technology and may speed your development (and reduce the chance of new programming errors).
Build in support to use a proxy. Consider using the SOCKS program to ensure that the code is firewall-friendly.
Make sure that good logging is performed. This includes logging connections, disconnects, rejected connections, detected errors, and format problems.
Build in a graceful shutdown so that the system operator can signal the program to shut down and clean up sensitive materials. Usually, this process means trapping the TERM signal and cleaning up afterwards.
Consider programming a "heartbeat" log function in servers that can be enabled dynamically. This function will periodically log a message indicating that the server is still active and working correctly, and possibly record some cumulative activity statistics.
Build in some self recognition or locking to prevent more than one copy of a server from running at a time. Sometimes services are accidentally restarted; such restarts may lead to race conditions and possibly the destruction of logs if the services are not recognized and are stopped early.
Don't write a new protocol. It's not easy to write a good network protocol, especially one that provides adequate security for authentication and authorization. Just as most cryptosystems devised by non-cryptographers are weak, most network protocols devised without expert consultation are flawed. Before you set out to write a new network protocol, see if a tried-and-true protocol already exists that can serve your needs.
If you must write a new protocol, don't write an asymmetric protocol. In an asymmetric protocol, a small client request results in a large server response. These kinds of protocols can make it easy to perform denial of service attacks on the server. This is of particular concern with connectionless services. Instead, write a protocol in which the amount of data exchanged is roughly equal on each side, or where the client is forced to do more work than the server.
Don't make any hard-coded assumptions about service port numbers. Use the library getservbyname() and related calls, plus system include files, to get important values. Remember that sometimes constants aren't constant.
Don't place undue reliance on the fact that any incoming packets are from (or claim to be from) a low-numbered, privileged port. Any PC can send from those ports, and forged packets can claim to be from any port.
Don't place undue reliance on the source IP address in the packets of connections you received. Such items may be forged or altered.
Don't require the user to send a reusable password in cleartext over the network connection to authenticate himself. Use either one-time passwords, or some shared, secret method of authentication that does not require sending compromisable information across the network.
Consider using this approach: The APOP protocol used in the POP mail service has the server send the client a unique character string, usually including the current date and time. The client then hashes the timestamp together with the user's password. The result is sent back to the server. The server also has the password and performs the same operation to determine if there is a match. The password is never transmitted across the network. This approach is described further in the discussion of POP in Chapter 12 ("Securing TCP and UDP Services") of Practical Unix & Internet Security, 3rd Edition.
Be sure to check back here next week for tips on writing SUID/SGID programs.
 This string is usually referred to as a nonce.
Note that the hash must not be reversable, or else the plaintext password could be divulged to an attacker monitoring the network.
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